devupstream: fix handling of SRC_URI

devupstream: fix handling of SRC_URI

As the class handler runs before overrides are actually applied we need
to check both SRC_URI:class-devupstream and SRC_URI, otherwise the
automatic assignment of S="${WORKDIR}/git" for git repositories does
not work if the base recipe uses http: but :class-devupstream has a git://
entry instead.

Also, there's no need to set S:class-devupstream, we can just assign to
S here.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>